As a developer for developing electrostatic latant image in electophotography, so-called two-component developer, which consists of a toner and a carrier, has widely known. The carrier is contained for causing the toner to be charged by friction in polarity suitable for developing operation.
Concerning the method for developing the toner, which is carried by carrier, transferred onto image-transfer sheet, and fixed onto the sheet, a pressure fixing method, especially low-pressure fixing method has been noted, because the method does not necessarily require heating for the fixing process. Moreover, the method can be ready for use in a short duration of time. As a toner suitable for the above-mentioned developing method, a capsule toner has been developed.
The following production methods of capsule toner have been proposed, for instance, spray-dry method, interface polymerization method, core-cellvation method, phase separation method, in-situ polymerization method, (these methods are disclosed in the U.S. Pat. Nos. 3,338,991, 3,326,848 3,502,582, etc.)
The capsule toner comprises a core (consisting of, as main components, coloring agents such as carbon black and magnetic powder, fixing resin, and binder such as, wax) and a shell (formed around the core and consisting of, as a main component, resin), wherein the shell is broken when a pressure is applied. The capsule toner has been regarded to be suitable for the pressure-fixing process, because the toner does not require high pressure for fixing operation and is superior in fixability. The conventional capsule toner, however, has not satisfactorily functioned regarding the essential characteristics required as toner.
As an example of such toner, so-called solid core capsule toner, of which core consists of solid wax, has been disclosed in the U.S. Pat. No. 4,259,426. Since the capsule toner of solid core type does not require high pressure for fixing process, it can be easily fixed. On the contrary, specific luster remains on the image after the fixing process, deteriorating the image quality. In addition, the fixing strength is weak, therefore, the image is damaged or smear may be generated when the developed paper is folded or rubbed. The capsule toner of this type has these problems not being solved.
As another example of the capsule toner, so-called liquid core capsule toner, of which core consists of a solvent, a resin for fixing, and a pigment, both which are dissolved in the solvent, has been disclosed in the U.S. Pat. No. 4,442,194. A schematic structure of the liquid core capsule toner is shown in FIG. 5. In the liquid core capsule toner, a shell 3 is broken by applied pressure, and a liquid resin for fixing process oozes out from the capsule and covers the pigment 2 to fix. When a capsule toner of liquid core type is used, the luster, which appears when a solid core type capsule toner is used, does not generally appear on the image after the development process, and excellent image quality can be obtained.
However, when the fixing process is attempted under a low pressure, the fixability is insufficient. Defective cleaning or filming, on the surface of photosensitive sheet may be occur. In addition, when the applied pressure is only enough to break the shell and the surface of the transfer sheet is rough, the toner enters into the clearance between paper fibers, thereby the toner particles not yet fixed, adheres onto the paper causing so-called toner stain.
On the other hand, the capsule toner containing liquid in its core is generally obtained in a shape of sphere having a small surface area. Therefore, since the surface area of the toner is small and the toner does not cause enough friction force when friction is applied to the toner and carrier in order to charge the toner, the electric charge is low and the charging operation does not smoothly start, thereby the toner easily scatters.
In addition, when spherical toner is subjected to running test using a PPC copying machine, which employs blade-cleaning process, toner particles cannot be completely cleaned off from the photoconductive drum and fuse and adhere onto the photoconductive drum when the blade cleaner wipes off the toner. The toner adhered onto the photoconductive drum is charged and developed in the following developing process, thus generating black stripes on the image. These types of toner also have such disadvantages that: since mechanical strength of the toner is low, the capsule is broken due to friction or impact caused by the carrier when stirring the developer; flowability of the developer is lowered; aggregation or clogging of the developer occurs, preventing smooth developing operation.